Electrolytic titrator



June 3, 1969 Filed la 4, 1964 M. L. ROBINSON ELECTROLYT IC '1' I TRAI'QR I Maw cmrex L J [4/64 i i/V65 MAX/M Z Oil RANGE MAX/MUM U WWW/T Sheet BWMM June 3, 1969 M. L. ROBINSON ELECTROLYTIC TITRATOR ql I I l I l i I I I I I I I l I l l w I I I I l I I'J m m im a I I l I u I l 0 l l I l l I I l l I l t I I l TVG 0 a W W J lllllJ e v. m m M J m \B n W m T t m v u r F Filed May 4, 1964 lrrae gw June 3, 1969 M. ROBlNSON ELECTROLYTIC TITRATOR Sheet Filed May 4, 1964 NM. R 0 %%m 4 4 W m United States Patent Oflice 3,448,031 Patented June 3, 1969 US. Cl. 204-195 19 Claims ABSTRACT OF THE DISCLOSURE Apparatus for the continuous electrolytic titration of a flowing fluid stream. The average of pulses of electric current generating the titrating agent serves as an indication of the instantaneous concentration of titratable reagent in the flowing stream. Means are provided to confine .the flowing stream to movement past three inline electrodes comprising, from the electrode furthest upstream, a reference electrode, a generating electrode, and a sensing electrode. The titrating agent generating electrode and the reference electrode are connected to a source of direct current through a switching means and the generator electrode and the sensing electrode are connected in a continuously operative sensing circuit connected to a control means. The control means controls the switching means in accordance with the concentration of titrating agent sensed by the sensing circuit. To prevent the generation of titrating agent in the switching circuit, the potential between the sensing electrode and the generating electrode is always below the potential necessary to generate the titrating agent. Alternative embodiments of a titration cell are provided to accommodate titratable reagents which are both non-hydrolyzing and those readily soluble in electrolytic solutions.

This invention relates generally to titration systems, and more particularly to new and improved electrolytic titration equipment employing novel titration cell designs and electrical circuitry for providing automatic and substantially continuous titration. In this connection, the ments in titration apparatus of the type set forth in my present invention is more specifically directed to improveprior Patent No. 3,248,309.

In the chemical arts, a wide variety of titration systems have been used for determining the concentration of a reactive agent in a sample fluid by way of electrolytic generation of specified titrating agents.

Generally, the titration systems of the prior art use relatively complex combinations of electrical circuitry and titration cell design to determine the concentration of the reactive agent in the fluid being monitored. These titration cells usually have separated compartments which house isolated electrodes provided with separate filtering and clearing arrangements. Such cells also require an appreciable amount of electrolytic solution for their operation and are, therefore, relatively bulky and not readily transportable in their assembled form. In addition, the relatively large volume of electrolytic solution reduces the ability of such titration cells to respond rapidly to sudden changes in concentration of the reactive agent. Furthermore, such cell designs usually require an appreciable period of time to stabilize after the addition of new solutions, or following a period of inoperation. Moreover, such cell designs generally do not provide means for continuously monitoring the concentration of a reactive agent present in a fluid, are capable of operation only over a relatively narrow range of titration currents, and are relatively insensitive to reactive agent concentrations of less than one part per million in a fluid sample under test.

In view of the foregoing, the titration apparatus of the prior art have proven generally unsatisfactory for monitoring processes such as those required in the bandling of high propellant fuels, where it is mandatory for human health to be able to continuously detect the presence of and rapid changes in concentration of dangerous noxious vapors over a wide range from one-tenth part per million to at least 500 parts per million in fluid samples. In my aforementioned prior Patent No. 3,248,- 309, there is disclosed a titration system employing a simplified control circuit in combination with a novel titration cell and elect-rode design which is extremely compact and rugged and requires less than two milliliters of electrolytic solution for proper operation. The latter titration apparatus provides automatic and continuous titration which responds rapidly to changes in the concentration of a reactive agent over a wide range extending from one-tenth part per million to at least 500 parts per million in the fluid being monitored.

The present invention provides additional improvements over the automatic titration systems of the prior art and of my prior Patent No. 3,248,309 by further simplifying titration cell design and utilizing a novel electrical system. The combined effect of this new cell design and electrical system is even greater, sensitive to rapid changes in concentration of a reactive agent and monitoring capabilities over an even wider range of concentrations with a single titration system.

Accordingly, it is an object of the present invention to provide a new and improved titrator which overcomes the above and other disadvantages of the prior art.

Another object is to provide a new and improved automatic titrator for continuous titration.

A further object of the invention is the provision of a new and improved titrator capable of titration over a wide range of titration currents from approximately peak microamperes to at least 200 peak milliamperes.

Still another object is to provide a new and improved titration system characterized by reduced response time to rapid changes in reactive agent concentration and, hence, capable of enhanced sensitivity in all titration ranges.

Yet another object of the present invention is the provision of a new and improved titration system having relatively long generating current on time for measuring low concentrations of a reactive agent, and, hence, capable of extremely high sensitivity in low ranges.

A still further object is .to provide a new and improved amperometric sensing and generation system for a reducible titrating agent in an electrolytic titrator.

Still another object is the provision of a new and improved polarized electrode amperometric system in an automatic titrator.

Yet another object is to provide a new and improved titrator requiring only three electrodes for the amperometric sensing and generation system.

A still further object of this invention is to provide a new and improved automatic continuous titrator wherein generation of titrating agents due to sensing circuit triggering alters the electric field in the sensing circuit to further enhance the full on state of the titrating agent generation circuit.

Another object is the provision of a new and improved titration system which obviates the need for mechanical relays and thus minimizes overshoot in titrating agent generation due to inertia of switch movement.

A still further object is to provide a new and improved continuous automatic titrator which is more economical,

more reliable, more stable and more accurate in operation.

The above and other objects and advantages of this invention will be better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings of illustrative embodiments thereof, and wherein:

FIGURE 1 is a combined electrical schematic and block diagram of a generalized system for an automatic titrator in accordance with the present invention;

FIGURE 2 is a schematic diagram of a presently preferred embodiment of the electrical circuitry employed by the titrator of the present invention;

FIGURE 3 is a graph of generating current vs. time for the high and low ranges of my improved titrator;

FIGURE 4 is a longitudinal sectional view of one embodiment of a titration cell in accordance with the invention and which is particularly suited to titration of nonhydrolyzing reactive agents;

FIGURE 5 is a fragmentary sectional view, taken substantially along the line 5'-5 in FIGURE 4, and illustrates the details of some of the titration cell components; and

FIGURE 6 is a longitudinal sectional view of another embodiment of a titration cell in accordance with the present invention, and illustrates a cell configuration which is particularly suited to the titration of reactive agents which are readily soluble in electrolytic solutions.

Referring now to the drawings, and particularly to FIGURE 1 thereof, there is shown a schematic representation of the electrolytic titrating system of the present invention. In this connection, the titration cell iself is denoted generally by the reference numeral 10, whereas the electrical control circuitry for the titrator is denoted generally by the reference numeral 11. A presently preferred embodiment of the control circuitry 11 is illustrated in FIGURE 2 of the drawings, and this same control circuitry may be used with either of the specific embodiments of the titration cell 10 illustrated in FIGURES 4 and 6 of the drawings and which will subsequently be hereinafter described.

The titration cell 10 includes an electrolytic solution (omitted for simplicity from the schematic representation of FIGURE 1) containing ions of an appropriate titrating agent. The titrating agent is a substance which combines in a known proportion with a selected reactive agent in the fluid sample introduced into the electrolytic solution during the monitoring process. By way of example, bromine may be used as the titrating agent in the titration cell 10, the known or determinable rate of generation of bromine in solution providing a means for determining the concentration of a reactive agent in a fluid which is sampled and passed through the electrolytic solution.

Briefly, to determine the concentration of the reactive agent, such as byproducts of sulphur or fuel vapor containing hydrazine, the ions of the titrating agent in the electrolytic solution are electrolyzed to generate a predetermined concentration of the titrating agent in the solution. The predetermined concentration of the titrating agent is arrived at by amperometric sensing means which sense the concentration of the titrating agent to produce a sensor current control signal. The latter control signal is, in turn, used to regulate the generation of the titrating agent. Hence, in response to additions to the electrolytic solution of a reactive agent which combines with and reduces the concentration of the titrating agent below a minimum predetermined level, new and improved means responsive to the sensor current control signal are energized for coulometrically generating additional quantities of titrating agent to automatically compensate for the sensed reduction in concentration. By continuously monitoring the flow of electrical current in the circuit for generating the titrating agent, the concentration of the reactive agent at any time is readily determinable.

To provide means for determining the concentration of a reactive agent in a fluid which is monitored by passing samples through the aqueous electrolytic solution, the titration cell 10 in FIGURE 1 includes an electrode assembly for generating the titrating agent from solution and sensing the concentration of the titrating agent. The electrode assembly includes a reference electrode R, a generator electrode G and a sensor electrode S. The generator and sensor electrodes are spaced from each other in the flow path of the sample fluid introduced into the titration cell the term reference electrode is used in this application in its generic sense, and indicates only the reference point from which the electrical potential of the generating electrode is determined. The term is thus not intended to be limited to the class of reference electrodes known as standard electrodes. The sample fluid bubbles through the electrolytic solution in the titration cell, where the reactive agent in the fluid combines with the titrating agent, and then passes upward through the generator and sensor electrodes in succession.

A D.C. power source 13 is utilized to apply a voltage across the generator and sensor electrodes. A resistor 15, which may be variable, is in series with the D.C. source 13 and sensor electrode and limits the maximum current flow in the sensing circuit. The sensor electrode S is maintained at a negative potential with respect to the generating electrode G, and the potential difference therebetween is below the 1.1 volt threshold level required to generate hydrogen at the sensor electrode, the latter being a cathode of a conductive, corrosion resistant material such as platinum, carbon or the like.

The D.C. source 13 also applies a voltage across the generating and reference electrodes such that the reference electrode is maintained at a negative potential with respect to the generator electrode, and this voltage between the electrodes G and R exceeds the hydrogen overvoltage level so that hydrogen gas is actually generated at the reference electrode. However, a normally open switching control 17 is in series with the electrodes G and R to normally prevent application of voltage from the D.C. source 13 and thereby prevent the generation of titrating agent by the generating circuit. A series variable resistor 19 limits maximum current flow in the generating circuit in the same manner as the resistor 15 limits current in the sensing circuit and, to this end, the two variable resistors may be ganged together for common control.

An appropriate recording meter 21, preferably of the ammeter type, is also in series with the electrodes G and R to measure the average value of generating current and, hence, provide a measure of the reactive agent concentration in the sample fluid introduced into the titration cell 10.

It will be apparent in FIGURE 1 that the series resistance 15 and the impedance between the electrodes S and G essentially form a potential divider for the total voltage applied by the D.C. source 13. However, for electrodes of a given size and electrolytic solution of a given electrical conductivity, the impedance and current flow between the electrodes S and G will be a function of the amount of titrating agent, e.g., bromine, supplied by the generator electrode and circulated by the electrolytic solution to the sensor electrode. Hence, when there is a deficiency of titrating agent in the electrolytic solution, the current flow between the electrodes S and G is relatively low, and the impedance and voltage across these electrodes is relatively high. On the other hand, when there is an excess of titrating agent in the electrolytic solution, the current between the electrodes S and G increases and the S-G impedance and voltage decreases.

Accordingly, an appropriate control signal means 23 shunts the electrodes S and G and is adapted to develop a control signal output whenever the S-G voltage rises above a predetermined level indicating a deficiency of titrating agent. The electrical output from the control signal means 23 triggers the normally open switching control 17 to close the latter and thereby activate the titrating agent generating circuit by applying the necessary voltage across the electrodes G and R.

The operation of the generalized system of FIGURE 1 is as follows. The generator electrode G is essentially a common electrode to both the sensing and generating circuits. Assuming that the titration cell is put in operation with fresh electrolyte, no significant concentration of the titrating agent bromine will be present in the electrolyte. However, bromide and hydrogen ions will be present in the solution in abundance.

Since the potential difference between the sensor and generator electrodes is below the hydrogen over-voltage level, a polarizing film of molecular hydrogen forms on the electrode S to inhibit current flow between the sensor and generator electrodes. Hence, the impedance between the electrodes S and G rises. Since, as previously indicated, the S-G impedance is in series with the current limiting resistance 15, which together form a potential divider, the increase in S-G impedance causes a corresponding increase in the voltage across the electrodes S and G. When the S-G voltage rises above a predetermined level (indicative of a bromine deficiency), the control signal means 23 produces an electrical output which activates the switching control 17 to apply voltage from the D.C. source 13 across the electrodes G and R and thus allow titrating agent generating current to flow therebetween. As a film of molecular bromine builds up on the generator electrode, the generator electrode is polarized. This polarization of the generator electrode, in turn, tends to further increase the impedance and voltage across the electrodes S and G in the sensing circuit. The result of the latter is to further enhance the full on switching action of the switching control 17.

The polarizing film of hydrogen which is formed on the sensor electrode S is adapted to be rapidly removed by titrating agent bromine in the region of the sensor electrode and to thereby vary the degree of polarization of the sensor electrode. Hence, when circulatory fluid flow in the titration cell 10 moves bromine generated at the generator electrode to the sensor electrode, the sensor electrode is partially depolarized, the sensing circuit current increases as a result thereof, and the 8-6 impedance and voltage decrease to a level whereby the control signal means 23 fails to hold the switching control 17 in the closed state. Thus, the flow of current between the electrodes G and R in the generating circuit, and hence the generation of titrating agent, is abruptly interrupted.

As the concentration of the titrating agent is reduced, as by bubbling out of the electrolytic solution during blank operation or by combination with a reactive agent when a fluid sample is introduced, the sensor electrode is polarized to a greater degree, and the generating circuit is again activated to provide additional quantities of the titrating agent. The entire titration system continues to cycle in this manner, with the generating current being turned on and off to generate discrete quantities of the titrating agent in accordance with the magnitude of the demand produced by the reactive agent and sensed by the sensing circuit as a reduction in titrating agent concentration. Since the generating current essentially supplies only enough titrating agent to satisfy this demand of the reactive agent, the generating current is a direct measure of the concentration of the reactive agent and is so recorded by the meter 21.

As will be hereinafter described in connection with the specific titration cells of FIGURES 4 and 6, the compact structural arrangement of the electrodes in the titration cell functions on a minimum quantity of electrolytic solution. Hence, the electrical system is capable of responding very rapidly to changes in concentration less than one hundredth part per million in the fluid sample.

Although some of the bromine generated at the electrode G is electrolytically reduced a tthe sensor electrode S, this bromine is coulometrically generated solely by the sensing current flowing between the electrodes G and 5, rather than by the generating current flowing between the electrodes G and R. Hence, the sensor electrode S uses only the bromine generated by its own current and no more, and thus the sensing circuit does not affect the concentration of the titrating agent in the electrolytic solution.

The current limitation provided by the series resistance 19 in the generating circuit is particularly important in providing for stable reference operation (blank operation) of the titration system in the absence of a reactive agent in the electrolytic solution, as well as maintaining the sensitivity of the system to small changes in concentration which would otherwise be masked if unnecessarily large concentrations of the titrating agent were generated.

Referring now to FIGURE 2 of the drawing, a specific embodiment of the new and improved titration circuitry is shown. The D.C. source 13 may consist of a battery 26. Alternatively, D.C. power may be supplied from an appropriate A.C. source which has been suitably rectified and regulated. In this latter connection, AC. voltage is supplied through a stepdown transformer T1 to a diode D1 whose rectified output is, in turn, filtered by a ripple filter R1, C1 and regulated by a Zener diode D2. Power on-off selection for application of operating voltage to the balance of the electrical system is accomplished by a multiple position series switch SW1.

The system of FIGURE 2 is designed for multiple range operation and, hence, the series current limiting resistance 15 consists of a bank of sensing current limiting resistors R2 through R8 in order of successively increasing resistance. When the resistor R2 is connected into the circuit, maximum sensing current is allowed for high range operation, whereas when the resistor R8 is connected into the circuit, minimum sensing current is allowed for low range operation.

Similarly, the series resistance 19 in the generating circuit consists of a bank of resistors R102 through R108 in order of successively increasing resistance. The low value resistor R102 permits higher generating currents for generating higher concentrations of the titrating agent, Whereas the largest resistor R108 limits the generating current to its lowest peak value for producing the titrating agent at a lower rate.

A bank 25 of resistors R202 through R208, in order of successively increasing resistance, provides a plurality of shunts for varying the range of the ammeter 21. In this connection, all of the resistor banks 15, 19 and 25 are ganged together for common control by a single titra- =tion range selector switch (not shown) which simultaneously sets up the system for proper maximum sensing current, peak generating current, and meter range for each selected titration range.

Resistors R10, R11 and condensers C2, C3 are connected between the bank of shunts 25 and the ammeter 21 to filter the current passing through the meter and provide a stable average reading.

The voltage from the D.C. source 13 is applied across a variable potentiometer R12. The potentiometer R12 permits fine adjustment of the current through the range resistors R2-R8 of the sensing current limiting bank 15 so that the predetermined level of titrating agent required to activate the control signal means 23 may be set for any range.

The control signal means 23 includes a PNP transistor TR1 with its base-emitter circuit connected across the generator and sensor electrodes. A diode D3 is connected as a non-linear impedance between the emitter of the transistor TR1 and the positive side of the D.C. power source 13 to limit the gain of the transistor TR1 at low current levels and thus prevent quiescent current from flowing through the transistor when it is not desired to activate the control signal means 23. This is especially important at elevated operating temperatures.

The generating circuit switching control 17 includes an NPN transistor TR2 connected in the conventional common emitter configuration. The base-emitter circuit of the transistor TR2 is shunted by a resistor R13 which sets the bias, and hence the state of conductivity, of the transistor TR2 when current flows through the latter resistor. The load in the collector circuit consists of the meter 21, shunted by resistance bank 25, in series with the reference electrode R. The input current to the base of transistor TR2 is through the resistance bank 19 of the generating circuit.

Let us assume that the range selector switch has been set to the next to lowest concentration range for the titrator. This connects the current limiting resistance R7 into the sensing circuit, the resistor R107 of the bank 19 into the generating circuit, and the resistor R207 as a shunt into the meter circuit.

When the concentration of titrating agent is low, as in starting the instrument, a polarizing film of hydrogen forms rapidly on the sensor electrode and thereby causes the impedance and voltage between the electrodes S and G to rise. This causes a corresponding increase in the base-emitter voltage of the transistor TR1, i.e., an increase in current into the base of TR1, until finally the high impedance at low current levels of the diode D3 is overcome and the transistor TR1 begins to conduct electrical current.

When the transistor TR1 becomes conductive, electrical current begins to flow through the emitter-collector circuit of the transistor TR1, the resistor R107 in the bank 19 and the resistor R13 which sets the bias on the transistor TR2. This causes the transistor TR2 to become conductive and thereby allows current to flow in the generating circuit between the electrodes G and R.

It will be observed that the resistor R13 and the iresistor R107 in the bank 19 form a potential divider and, hence, the relative values of the resistors R13 and R107 establish the degree of conductivity of the transistor TR2 :and thus the maximum current for any given titration range Hence, as resistors are substituted for the resistor R107 in the bank 19 by operation of the titrator range selector switch, the base-emitter bias voltage of the transistor TR2 and the maximum value of peak generating current that can flow between the generator and reference electrodes are varied for each range.

As generating current begins to flow between the reference .and generator electrodes, bromine is formed on the surface of the electrode and hydrogen is formed at the reference electrode R. The localized increase :in bromine concentration around the generator electrode further increases the impedance and voltage between the electrodes G and S in the sensing circuit, with a consequent increase in the base-emitter voltage of the transistor TR1. This enhances the full on condition of the transistor TR1 acting as a switch.

The generating current remains on until enough of the bromine reaches the sensor electrode by circulation of electrolytic solution from the region of the generating electrode to the sensor electrode region. When this occurs, more current flows to the electrode S, thereby reducing current flow into the base of the transistor TR1. The genera-tor current is thus shut off when the transistors TR1 and TR2 return to their non-conductive states. There is no inertia in the electronic circuit; on and olf operation of the transistor switching system is relatively rapid. As the bromine is carried to the sensor electrode, response to decreased voltage in the sensing circuit is very fast and causes interruption of the generating current flow with minimum overshoot. In this connection, the diode D3, by virtue of its high impedance at low current levels, has a modulation effect which complements the current limiting action of the resistor bank 19 to minimize overshoot in the lower titration ranges. Hence, :at low current levels, the diode D3 limits the bard-on action of the transistor TR1 to that compatible with the circulation time constant of the titration cell.

The titration cell 10 is deliberately designed so that solution movement is from the generator electrode to the sensor electrode, and these electrodes G and S are spaced closely together. Hence, interruption of the generating current occurs before the solution has been brought to the desired bromine level. As mix-ing progresses, the concentration of titrating agent is found insufficient to carry the sensing current, and the generating current is again turned on by the transistors TR1 and TR2. The servo system thus operates by pulsed current control to maintain the concentration of titrating agent of the predetermined level set by the resistors in the sensing current bank 15.

As the sensing current limiting resistance in the circuit of FIGURE 2 decreases in value, a greater proportion of the voltage tapped from the potential divider R12 appears across the electrodes S and G. Therefore, it follows that, as the sensing current limiting resistance decreases in value, the transistor TR1 will remain conductive in the face of increased sensor electrode currents and hence increased concentrations of the titrating agent in solution. Thus, with low resistance in the sensing circuit, the titration cell 10 will operate with more bromine in the electrolytic solution and hence greater efiiciency for oxidation of higher concentrations of a reactive agent present in the sample gas introduced into the cell. On the other hand, it will be apparent that extreme sensitivity can be achieved by the use of high resistance in the sensing circuit which causes the transistor TR1 to become conductive in the presence of much lower concentrations of the titrating agent in the electrolytic solution.

Location of the reference electrode R in the opposite direction from the generator electrode G than the sensor electrode S prevents excessive positive feedback from the reference electrode to the sensor electrode which would tend to lock the transistor TR1 in a state of continuous conduction.

Referring now to FIGURE 3, the pulsed character of the generating current for high and low ranges will be apparent. In this connection, it has been discovered that sensitivity can be enhanced by limiting the maximum generating current available so that, at very low input of titratable material, there is a substantial proportion of on-time during the titration process. Hence, for the higher ranges, the generating current can, if required, be turned on to a higher maximum level each time the transistor TR2 becomes conductive. Thus, a generating current pulse for the high range is capable of producing a greater quantity of titrating agent than a generating pulse of the same duration for the low range. This graduated arrangement permits high sensitivity to changes in reactive agent concentration in all ranges and, at the same time, permits an extremely wide range of titration currents from as little as 100 peak microamperes to well in excess of 200 peak milliamperes.

Referring now to FIGURES 4 and 5 of the drawings, there is shown a titration cell 30 which is particularly adapted to the titration of substances such as sulphur com-pounds which are relatively insoluble in aqueous solutions. To provide such selective titration, the titration cell 30 includes means for thoroughly mixing the sulphur compound with the electrolytic solution to completely combine with the titrating agent in the solution.

The titration cell 30 includes an outer container 32 which provides a reservoir for an electrolytic solution 36. The solution 36 contains ions of .an appropriate titrating agent such as bromine or the like. The electrolytic solution may be any acidified bromine solution. A presently preferred example is hydrobromic acid which will not salt up or clog bubblers or other apparatus, is completely volatile, maintains more uniform concentration in use, and induces bromine to be more reactive in the titration process.

Sealing the open end of the container 32 is a stopper member 38. To introduce the electrolytic solution and sample fluid into the container 32, an inlet tube of plastic, such as polyethylene or the like, extends through the stopper 38 into the container 32. Inserted in a gas-tight- 9 fit into the lower end of the inlet tube is a plastic tube 42 having a porous bubbler 45 fusion welded to the lower end of the tube 42.

The bubbler 45 is a wettable frit of glass, plastic or the like. The outer shape of the bubbler 45 may be any desired configuration which will not provide a trap for rising froth or gas bubbles. In this connection, the lower end of the bubbler 45 is provided with a tapered surface 46, preferably in the form of an inverted cone, to direct bubbles upward along the surface of the bubbler and prevent the occurrence of vapor lock which would cause erratic operation.

The upper end of the bubbler 45, adjacent the lower open end of the tube 42, is provided with a gas distribution cavity 47. The cavity 47 extends deeply into the bubbler 45 and has a conical shape approximating the outer surface configuration of the bubbler but with a slightly sharper taper. The effect of the cavity 47 is essentially to provide a uniform mean path through the bubbler 45 for the sample gas introduced through the inlet tube 42.

A titrating agent generating electrode 49 is mounted on the outer surface of the bubbler 45 and is preferably fused thereto. The generator electrode 49 may be a single turn of platinum wire, a layer of gauze or the like.

A sensor electrode 53, in the form of a large diameter coiled loop of platinum wire is mounted upon the lower end of the inlet tube 40 and is held thereto by a plurality of tack welds. The sensor electrode 53 is thus positioned above the bubbler 45 and generator electrode 49 in the path of fluid froth which contacts and then rises above the generator electrode.

A reference electrode 55, which may take the form of a single turn of platinum wire, is positioned below the generator electrode 49 along the bubbler 45. The reference electrode may be held in place by fusing the electrode to the bubbler.

In operation, sample gas entering the bubbler 45 through the inlet tubes 40, 42 and cavity 47 bubbles out of the bubbler and into contact with the generator electrode 49. The gas then bubbles upwardly through the solution confined by .a cylindrical sleeve 57 surrounding the electrode assembly. 'As the gas bubbles upwardly through the solution, it makes active contact with and scrubs the surface of the sensor electrode 53.

A substantially cylindrical cell cup '63, having an upper expanded portion 64, surrounds the sleeve 57 and inlet. tube 40. In this connection, the sleeve 57 is provided with an anuular shoulder 65 in .abutment with the inner wall surface of the cell cup 63. The shoulder 65 is provided with a plurality of bores or passages 66 at spaced locations about its periphery. Each of the passages 66 communicates the space between the cell cup 63 and sleeve 57 with the bubbler and electrode region on the opposite side of the sleeve 57. Fluid communication is thus provided between the cell cup and the reaction region.

A plastic cap 68 is fitted to the upper open end of the cell cup 63 and is provided with a plurality of gas vent holes 70. The cap 68 is also provided with a central clearance aperture through which the inlet tube 40 passes.

The lower open end of the cell cup 63 is closed by a plastic cap or plug 72. The plug 72 is also provided with a central aperture 73 and supports therein a loose fitting flow limiting pin or rod 75 which provides restricted communication between the inside of the cell cup and the outer reservoir of electrolytic fluid 36.

When bubbles of solution move upwardly past the sensor electrode 53 and up through the sleeve 57, these bubbles burst in the upper expanded portion 64 of the cell cup. The electrolytic solution which is carried with the bubbles drips down the sides of the cell cup 63 between the cell cup and the sleeve 57, through the bores in the shoulder 65, and hence returns to the bubbler 45. The gases which separate from the solution in the upper expanded portion 64 of the cell cup pass out of the cell cup through the vent holes in the cap 68 and then escape through an outlet tube 78 which extends through the stopper 38. All hydrogen gas generated at the reference electrode 55 escapes with the spent sample gas in this manner. Electrolytic solution lost by evaporation or by spattering through the vent holes 70 is made up by additional electrolytic solution 36 entering the cell cup 63 through the flow limiting arrangement in the plug 72 at the bottom of the cell cup.

The gas outlet tube 78- includes a diagonal slot 80 facing the stopper 38. Disposed within the lower end of the outlet tube 78 is a filter 82 of glass wool or the like to trap residual electrolytic solution and allow the latter to drip back down into the lower end of the container 32.

As previously set forth in connection with the electrical systems of FIGURES 1 and 2, the generator and sensor electrodes are coupled to electrical circuitry external to the titration cell to generate the titrating agent at the generator electrode and sense the concentration of the titrating agent by current flowing between the sensor electrode and the generator electrode. In order to provide the necessary electrical connections to the sensor, generator and reference electrodes, three insulated electrical conductors 85 are included within the container 32 and enter the cell cup through one of the vent holes 70 in the cap 68. A single conductor 85 is connected to each of the elec trodes 49, 53 and 55 and passes through a conduit 87, of plastic or the like, which extends through the stopper 38. The conductors 85 terminate in an appropriate external electrical connector 89 adapted for plug-in connection to appropriate electrical circuitry.

Due to the combination of the bubbler 45 and the generator and sensor electrodes 49, 53, respectively, along the path of the sample fluid which is bubbled through the electrolytic solution, automatic continuous titration is provided by the titration cell 30 of FIGURE 4.

Due to the compact bubbler and electrode arrangement, the continuous titration is provided within an extremely small working volume of electrolytic solution. Thus, when changes occur in the concentration of the reactive agent being bubbled through the electrolytic solution, a minimum of time is required for the generator electrode 49 to produce suflicient amounts of the titrating agent to return the sensor current to the predetermined level whereby the transistor TRl in FIGURE 1 becomes non-conductive. Moreover, the bubbler-electrode arrangement operating on a small volume of electrolytic solution, in combination with the circuitry of FIGURES 1 and 2, is highly sensitive to low concentrations of a reactive agent. In this connection, the system is capable of sensing concentrations of reactive agents as low as one hundredth part per million.

Referring now to FIGURE 6, there is shown a titration cell which is particularly adapted to the titration of reactive agents which are readily soluble in electrolytic solutions, such as fuel vapor containing hydrazine and its derivatives, As is well known in the art, hydrazine and its derivatives are compounds which hydrolyze in the presence of water and then react slowly with titrating agents such as bromine. Accordingly, it is necessary to minimize contact of the sample gas containing hydrazine with the electrolytic solution while providing maximum contact with the titrating agent itself. Such a cell is inherently insensitive to relatively insoluble reactive agents such as sulphur compounds.

The titration cell 100 includes a container 101 in which is disposed an electrolytic solution 102. The electrolytic solution 102 contains ions of a titrating agent such as bromine which combines with the hydrazine in known proportions to provide means for determining the concentration of the hydrazine present in a sample fluid which is bubbled through the electrolytic solution. Since hydrazine is readily soluble in the electrolytic solution, it is desirable to use a solution which has a low electrolyte concentration and, hence, a low electrical conductivity. By way of example, the electrolytic solution 102 may be an aqueous solution comprising 3% potassium bromide, 5% sodium citrate, and A of 1% citric acid.

The titration cell also includes an outer tubular member 104 which may be fabricated of polyethylene or the like. Disposed at the upper end of the tubular member 104 is a stopper 106. The stopper 106 includes three adjacent annular sections 108, 110, and 112 of decreasing radial dimension. The annular section 112 fits within an end of the outer tubular member 104. The annular section includes an annular slot 116 and is dimensioned to fit within a top member 114 to seal the open top of the container 101. Positioned within the annular slot 116 is an O ring 118 which provides an airtight seal between the stopper 106 and the top member 114. The annular section 108 rests on the top member 114 to support the stopper member 106 and the outer tubular member 104 within the cell. The tubular member 104 extends downwardly into the electrolytic solution 102, the lower open end 120 of the tubular member being adjacent the bottom of the container 101.

Immediately above the open end 120 of the tubular member 104 is a slot 122 in the surface of the tubular member. Due to the sample fluid which is pumped into the compartment defined by the tubular member 104 and the stopper 106, the electrolytic solution 102 rises within the tubular member to a point adjacent the upper edge of the slot 122. As will be subsequently explained, this provides a relatively small working volume of electrolytic solution in the region of the electrode assembly.

Supported by the stopper 106 and extending longitudinally within the tubular member 104 is a rod 124. The rod 124 may be fabricated of Teflon or the like and has a cylindrical end member 126 having a concave lower face 128 which is preferably in the shape of a cone.

Mounted within the tubular member 104 is a sleeve member 130 of a non-absorbent material, such as Teflon or the like, and including adjacent end sections 132, 134. The end section 132 has a larger volume than the end section 134 and tits tightly around the circumference of the member 126. In this manner, the end section 134 extends longitudinally within the tubular member 104 towards the bottom of the container 101 and terminates above the open end 120.

Disposed within the lower open end of the end section 134 is a glass plug member 136 having a bore 138 therein. Positioned within the bore 138 is a glass rod 140. By capillary action, the surface of the glass rod 140 is wetted to provide a communicating path for the electrolytic solution 102 into a chamber defined by the sleeve 130 and the end member 126. Accordingly, the electrolytic solution 102 rises within the sleeve 130 to a level substantially even with the level of the solution within the tubular member 104.

Mounted within the sleeve 130 is an inner tubular member 142 of glass or the like which extends longitudinally within the outer tubular member 104. The inner tubular member 142 is fixedly secured in coaxial alignment within the sleeve 130 by spacers 144, 146 which provide a pressure fit between the inner tubular member 104 and the end section 134 of the sleeve 130.

Disposed within the inner tubular member 142 is an electrode assembly 148 which includes three electrodes spaced from each other along the path traversed by the sample fluid as it is bubbled through the electrolytic so- 'lution. In this manner, the sample fluid containing the reactive agents combines with the titrating agent in the region of a generator electrode 150 which also functions as an anode in the sensing circuit. The reduction in concentration of the titrating agent is sensed by a reduction in current flow between the generator electrode and a sensor electrode through which sample fluid and titrating agent are bubbled.

To provide such operation in the titration cell shown in FIGURE 6, electrode assembly 148 includes the generator electrode 150, which preferably takes the form of a platinum wire formed into a coil mounted axially in the lower section of the tubular member 142. This generator electrode 150 is mounted axially within a coiled reference electrode 152. Spaced above the electrodes 150, 152, along the inner surface of the inner tubular member 142, is a sensor electrode 154. The sensor electrode 154 takes the form of a coiled platinum wire extending from a point immediately above the generator electrode 150 to the upper open end of the tubular member 142. Such an electrode arrangement provides maximum electrode contact with the bubbling sample fluid and the electrolytic solution, as well as proper interaction of the electric field around the generator electrode with the sensor electrode system.

To provide means for connecting the generator, reference and sensor electrodes 150, 152, 154, respectively, to electrical circuitry of the type shown in FIGURES l and 2, electrical conductors 158, and 162 are included. These electrical conductors are enclosed in plastic tubes which extend through the stopper 106 to an appropriate terminal connector 164 which is adapted to be coupled to appropriate external electrical circuitry. The terminal conductor 164 is fixedly secured to the stopper 106 by a screw 166 and is covered by a cap 168, of plastic or the like, which is suitably afiixed to the stopper 106.

To introduce the sample fluid containing unknown amounts of a reactive agent into the electrolytic solution of the titration cell, an inlet tube exteneds through a slot 172 in the cap 168 and stopper 106, downward into the outer tubular member 104, and into the sleeve 130 at a point above the plug 136 and reference electrode 152 and just below the generator electrode 150. The end of the tube 170 is diagonally cut such that sample fluid is emitted in an upward direction through the inner tubular member 142. The tube 170 is preferably composed of Teflon. In this manner, an inlet is provided for the fluid which is resistant to plugging by fuel vapor and is a non-wetting arrangement for bubbling the sample fluid through the electrolytic solution in the inner tubular member 142.

By virtue of the cooperating structural arrangement of the electrode assembly and the non-wetting bubbler provided by the tube 170, uniform velocity of fluid flow is maintained in the region of the sensor electrode 154 to insure that the sensor current is an accurate indication of the concentration of the titrating agent in solution.

The bubbles of sample fluid and electrolytic solution, in passing from the inner tubular member 142, strike the concave surface 128 of the end member 126 and burst. The electrolytic solution containing the titrating agent is splattered onto the inner surface of the sleeve 130 and drips downward between the latter sleeve and the inner tubular member 142 to the pool of electrolytic solution within the inner tubular member. The fluid in gaseous state passes from the chamber defined by the sleeve 130 and the end member 126 through an opening 172 in the end section 132 of the sleeve 130. The fluid then expands in a chamber defined by the outer tubular member 194 and the stopper 106 to create a pressure in the latter chamber which maintains the level of the electrolytic solution within the outer tubular member at a point adjacent the upper edge of the opening 122. The gas then bubbles through the opening 122 and through the electrolytic solution 102 to the surface thereof. There the gas passes from the electrolytic solution through an opening 174 in the top member 114 to the atmosphere.

The gas, in passing from the outer tubular member 104 through the opening 122, causes a pumping action to occur which not only maintains a circulation of the electrolytic solution within the container 101 and within the outer tubular member 104, but also allows electrolytic solution to be pumped through the plug 136 into the inner tubular member 142 to maintain the level of electrolytic solution therein. In this manner, controller circulation of the electrolytic solution within the titration cell is provided to 13 maintain uniform electrolyte concentration within the solution;

Both the titration cell 30 of FIGURE and the cell 100 of FIGURE 6 use only small working valumes of electrolytic solution. This enables rapid response of the titration cell to changes in the concentration of a reactive agent being bubbled through the cell, since only small amounts of the titrating agent need be generated to compensate for such changes in the concentration of the reactive agent.

It will be apparent from the foregoing that, while particular forms of my invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of my invention. Accordingly, I do not intend that my invention be limited, except as by the appended claims.

I claim:

1. Titration apparatus, comprising:

a porous bubbler;

a generator electrode carried upon the outer surface of said bubbler for generating a titrating agent from an electrolytic solution;

a sensor electrode for sensing the concentration of said titrating agent;

means for supporting said sensor electrode above said generator electrode;

a reference electrode carried upon the surface of said bubbler below said generator electrode;

means for developing a potential difference between said generator electrode and said sensor electrode which is below the threshold level necessary to generate hydrogen at said sensor electrode;

and means responsive to the magnitude of electrical current flowing between said generator electrode and said sensor electrode for simultaneously controlling the flow of electrical current between said generator electrode and said reference electrode.

2. Titration apparatus according to claim 1, including means for defining a flow path for a sample fluid through said electrolytic solution, said reference, generator and sensor electrodes being in said flow path.

3. In a titration system, the combination comprising:

an inlet tube for sample fluid;

a porous bubbler supported adjacent the lower end of said inlet tube, said bubbler having a substantially conical distribution cavity adjacent the lower open end of said inlet tube, said bubbler having the shape of an inverted cone at the lower end thereof for di verting gas bubbles upwardly along the surface of said bubbler;

a gauze generator electrode fused to the outer surface of said bubbler for generating a titrating agent from an electrolytic solution;

a sensor electrode in the form of a coil of metal wire for sensing the concentration of said titrating agent, said sensor electrode being supported by said inlet tube above said generator electrode;

a metal reference electrode fused to the outer surface of said bubbler below said generator electrode;

means for developing a potential difference between said generator electrode and said sensor electrode which is below the threshold level necessary to generate hydrogen at said sensor electrode;

and means responsive to the magnitude of electrical current flowing between said generator electrode and said sensor electrode for controlling the flow of electrical current between said generator electrode and said reference electrode.

4. A combination as set forth in claim 3, wherein said porous bubbler is of fritted glass.

5. In an electrolytic titration system, the combination comprising:

a container for housing a body of electrolytic solution containing ions of a titrating agent;

guide means within said container for directing a sample fluid along a controlled predetermined fluid path through said electrolytic solution and said container;

a single generator electrode supported within the portion of said container adapted to house said electrolytic solution and in said predetermined fluid path for generating said titrating agent from said electrolytic solution;

a single sensor electrode supported within said container and spaced above said generator electrode along said predetermined fluid path; a

a single reference electrode supported within said container and spaced below said generator electrode;

means for continuously applying a voltage between said generator electrode and said sensor electrode which is below the hydrogen over-voltage level;

switching means for selectively applying a voltage be tween said generator electrode and said reference electrode in excess of the hydrogen over-voltage level;

and control signal means responsive to the flow of electrical current between said generator electrode and said sensor electrode for selectively energizing said switching means.

6. A combination as set forth in claim 5 wherein said switching means and said control signal means are both semiconductor devices.

7. A titration system, comprising:

a container for housing a body of electrolytic solution containing ions of a titrating agent;

an inlet tube for sample fluid;

a porous bubbler supported adjacent the lower end of said inlet tube, said bubbler having a substantially conical distribution cavity adjacent the lower open end of said inlet tube, said bubbler having the shape of an inverted cone at the lower end thereof for diverting gas bubbles upwardly along the surface of said bubbler;

a gauze generator electrode fused to the outer surface of said bubbler for generating a titrating agent from said electrolytic solution; sensor electrode in the form of a coil of metal wire for sensing the concentration of said titrating agent, said sensor electrode being supported by said inlet tube above said generator electrode; metal reference electrode fused to the outer surface of said bubbler below said generator electrode;

substantially cylindrical sleeve means surrounding said bubbler, all of said electrodes and the lower portion of said inlet tube to define a predetermined fluid path for sample fluid introduced to said container, said generator and said sensor electrodes being in said predetermined fluid path;

means for applying a voltage between said generator electrode and said sensor electrode which is below the hydrogen over-voltage level;

switching means for selectively applying a voltage be tween said generator electrode and said reference electrode;

control signal means responsive to the flow of electrical current between said generator electrode and said sensor electrode for selectively energizing said switching means;

and means for indicating the magnitude of the electrical current flowing between said generator and said reference electrodes.

8. A combination as set forth in claim 7, wherein said switching means and said control signal means are both semi-conductor devices.

9. Titration apparatus comprising:

receptacle means providing a chamber to contain an electrolyte;

sensor, generator and reference electrodes fixed relative to said receptacle means in a position to be immersed in said electrolyte;

flow means for passing a sample fluid through said electrolyte in a predetermined direction, said sensor electrode being spaced from said generator electrode in said predetermined direction, said reference electrode being spaced from said sensor electrode in a direction opposite said predetermined direction;

a source of direct-current potential having one side connected to said generator electrode;

a'control device connected between the other side of said source and said reference electrode, said device being actuable to provide a low impedance from said source to said reference electrode, said source being poled to cause a titrating agent to be generated by the difference of potential created between said generator and reference electrodes;

potential means for continuously maintaining said sensor electrode at a predetermined potential relative to but different from that of the same generator electrode and of a polarity the same as that of said reference electrode;

and circuit means for continuously controlling said device while said sensor electrode is maintained at said predetermined potential, said circuit means controlling said device to vary the potential of said reference electrode relative to said generator electrode in a manner to increase the production of said titrating agent when the resistance between said sensor and generator electrodes increases and to decrease the production of said titrating agent when the resistance between said sensor and generator electrodes decreases, the average current through said control device thereby being directly proportioned to the concentration of the substance being detected, said circuit means being actuable continuously while a difference of potential is supplied Ibetween said sensor and generator electrodes, the potential applied to said reference electrode increasing the sensor-to-generator resistance until said titrating agent reaches said sensor electrode.

10. The invention as defined in claim 9, wherein said control device is a normally open switch, said circuit means being adapted to close said switch when the resistance between said sensor and generator electrodes exceeds a predetermined resistance, and to open said switch when the resistance between said sensor and generator electrodes falls below said predetermined resistance.

11. The invention as defined in claim 10, wherein said sensor electrode is positioned outside the space between said generator and reference electrodes.

12. The invention as defined in claim 11, wherein said flow means includes a first vertical tube to carry said sample fluid to an outlet position below the liquid level of said electrolyte and release it there, and a second vertical tube over said outlet to contain said electrolyte and said sample fluid in a confined space, all three of said electrodes being located in said confined space, said sensor electrode being located above said generator electrode, said generator electrode being located above said reference electrode, said sample fluid having a density less than that of said electrolyte.

13. The invention as defined in claim 12, wherein said sample-fluid is a gas and said electrolyte is a liquid.

14. The invention as defined in claim 13, wherein said source of potential has a positive side connected to said generator electrode and a negative side connected to said sensor electrode through said potential means, the same said source having a negative side connected to said reference electrode via said control device.

15. The invention as defined in claim 9, wherein said sensor electrode is positioned outside the space between said generator and reference electrodes.

16. The invention as defined in claim 9, wherein said flow means includes a first vertical tube to carry said sample fluid to an outlet position below the liquid level of said electrolyte and release it there, and a second vertical tube over said outlet to contain said electrolyte and said sample fluid in a confined space, all three of said electrodes being located in said confined space, said sensor electrode being located above said generator electrode, said generator electrode being located above said reference electrode, said sample fluid having a density less than that of said electrolyte.

17. The invention as defined in claim 16, wherein said sample fluid is a gas and said electrolyte is a liquid.

18. The invention as defined in claim 17, wherein said flow means includes a vent at the upper end of said second tube to allow said gas to escape therefrom.

19. The invention as defined in claim 9, wherein said source of potential has a positive side connected to said generator electrode and a negative side connected to said sensor electrode through said potential means, the same said source having a negative side connected to said reference electrode via said control device.

References Cited UNITED STATES PATENTS 3,272,731 9/ 1966 Hutchison et al 204- 3,337,440 8/1967 Nestor 204195 3,028,317 4/ 1962 Wilson et al 2041.1 3,061,773 10/1962 Ellison et a1 204-196 3,127,337 3/1964 Conger et al 204-496 3,131,133 4/1964 Barendrecht 204-1.1 3,131,348 4/1964 Taylor et al. 204195 3,154,477 10/1964 Kesler 2041.1 3,162,585 12/1964 De Ford et a1 204-195 3,234,117 2/1966 Rost et a1. 204-195 3,236,759 2/1966 Robinson 204195 3,248,309 4/ 1966 Robinson 2041.1 3,280,020 10/1966 Conger 204196 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. Cl. X.R. 

